Recently, Reverse Conducting-IGBT (RC-IGBT) with an Insulated Gate Bipolar Transistor (IGBT) and a diode formed on the same substrate has been in active development. One of the structural elements of the IGBT is a gate region extending in a trench. A p type base layer is etched to make a trench therein, and on the surfaces of the trench, an oxide film is formed as a gate oxide film. Thereafter, an electrode material is formed or located over the gate oxide film covering the trench surfaces, to form the trench as a gate region. An IGBT having the gate trench structure can make a channel portion of the resulting device denser than in the case of using the planar structure with a gate oxide film and a gate electrode formed thereover on the upper surface of the base layer, and thus achieving increased current density where the trench style gate is used.
However, the trench electrode structure also presents some issues. When a distance between adjacent trench electrodes is large, at the trench electrode near a terminal end portion of the device, the electric field is concentrated in the bottom of the trench when the device is in a reverse bias state, resulting in a reduction in the static breakdown voltage and risk of device destruction. In an attempt to ameliorate these issues, in trench electrodes near the terminal end portion and widely spaced trench electrodes, the bottom of the trench is covered with a diffusion layer having a higher dopant concentration in order to not generate a concentration of the electric field in the bottom of the trench.
In the RC-IGBT, however, since the IGBT and the diode are formed on the same substrate, there necessarily exists at least one location where the IGBT is close to the diode. A trench electrode of the IGBT is to be located in this close portion, where the concentration of the electric field occurs. When the diode is formed as a planar structure for the purpose of improving characteristics, the concentration of the electric field in the adjacent portions of the IGBT and the diode is avoided. When the diode structure is also formed inwardly of the substrate, a method for avoiding the concentration of the electric field in the close portion is also desired.